Outlook Attachment Size Limit: Maximum MB by Client, and How to Get a PDF Under It

Ask "what is the Outlook attachment size limit" and you will get four different answers, all of them correct. The number that stops you depends on how your account connects to Outlook, and on who is receiving the message.

Outlook attachment size limits at a glance

The rest of this guide explains where each number comes from, how to tell which one just rejected your message, and what to do about it.

Three limits, and the smallest one wins

Microsoft's own guidance is explicit that the smaller of the mailbox limit and the client limit is what applies. A 150 MB desktop client cap is irrelevant against a 35 MB mailbox policy — which is why raising the tenant limit is often the only thing that actually helps, and even then only for recipients who can accept it.

What each Outlook client allows

These are Microsoft's published figures for a Microsoft 365 mailbox whose policy has been raised to the maximum. They are ceilings, not what most accounts actually get.

Two details worth knowing:

• The 150 MB figure has a catch. Microsoft notes it is only reachable between users whose messages never leave its datacenters. The moment a message crosses to an external recipient, the 33 percent encoding increase applies and the effective maximum drops to 112 MB — before the recipient's own limit takes over.
• There is also a 250-attachment-per-message cap, which matters if you are sending a large number of small PDFs rather than one large one.

The 20 MB limit nobody mentions: Outlook's own client cap

If you added a Gmail, Yahoo, iCloud or ISP mailbox to Outlook desktop, the Microsoft 365 numbers above do not apply to you. Outlook 2013 and later apply their own 20 MB (20,480 KB) cap to Internet email accounts — POP3, IMAP and HTTP — counting every attachment in the message together. Exceed it and Outlook refuses before anything is sent:

The file you're attaching is bigger than the server allows. Try putting the file in a shared location and sending a link instead.

Microsoft's stated reason is to stop the client from repeatedly trying to upload attachments that most providers would reject anyway. Two consequences follow:

• The same wording appears on Exchange and Microsoft 365 accounts, but there it reflects the mailbox's maximum send size — 10 MB by default on Exchange Server — rather than the 20 MB figure. The message text does not tell you which limit you hit; the account type does.
• Microsoft documents a per-user registry value, MaximumAttachmentSize, under HKEY_CURRENT_USER\Software\Microsoft\Office\<version>\Outlook\Preferences, expressed in kilobytes, that raises or removes the 20 MB client check. It changes nothing on any server. Raising it to 30 MB for a Gmail account only moves the failure from Outlook's warning to a bounce from Google's 25 MB limit. Unless you know the provider's limit is higher, reduce the file instead.

Outlook.com versus Microsoft 365

These get conflated constantly, but they are different things:

Exchange Server on-premises is the tightest of all: Microsoft documents a 10 MB default for both maximum send and maximum receive size. If your work email rejects attachments that sail through from your personal Gmail, an on-premises or gateway limit is the usual reason.

Why the same PDF behaves differently in each client

Attachments are base64 encoded before transmission, which expands them by roughly a third. The web client's 112 MB figure is not arbitrary — it is the 150 MB ceiling with encoding headroom already subtracted.

So a "35 MB" mailbox policy is not 35 MB of file on your disk. After encoding, and after the message body, signature, and any inline images are counted, the practical file size is closer to 25 MB.

Reading the error: a warning before sending versus a bounce after

Where the rejection happens tells you whose limit you hit, and therefore what can fix it.

A bounce that arrives hours later is the recipient's server or a gateway in between; an immediate refusal is yours. Sending the message to yourself first is the quickest way to tell them apart.

What to do when the message is too large

Work through these in order:

1. Check whether it is really the file. Send the message to yourself first. If it bounces immediately, it is your outbound policy. If it bounces hours later, it was the recipient.
2. Compress the PDF locally. Open the compress tool, drop the file in, and reduce it to fit. Processing happens in your browser, so contracts, invoices, and statements are not uploaded anywhere. For choosing a level and checking the result, follow how to reduce PDF file size online for free.
3. Shrink the source rather than the output. If the PDF is a scan, rescanning at 200 DPI in grayscale beats compressing a 600 DPI colour file, and the result is usually both smaller and more legible. If you are not sure why the file is large in the first place, start with why is my PDF so big.
4. Send a link for anything that must not change. OneDrive and SharePoint links handle signed contracts and fillable forms that compression would damage.

If the document is sensitive — a contract, a statement, a medical record — the kind of tool matters as much as the setting. Online, desktop and AI PDF compression compared sets out which approach keeps the file on your device.

If you are on Microsoft 365, find out your real limit

Rather than guessing, ask whoever administers your tenant — or if that is you, check the transport configuration. The organisation-wide setting and per-mailbox settings can differ, and both cap below the client figures.

Raising the limit is a legitimate fix for internal file sharing, but it does nothing for external recipients. If you regularly send large PDFs outside your organisation, compression or a link is the answer, not a policy change.

Sending to or from another provider

The recipient's provider is the limit you cannot see. The published numbers for the other major services are in Gmail's 25 MB attachment limit and Yahoo, iCloud, Proton and Zoho attachment limits. If the file is going to a messaging app or an upload portal rather than an inbox, the rules are different again — PDF attachment size limits for email, messaging and online forms routes you to the right one.

Official sources

• Exchange Online limits — Microsoft's service description, including the default 35 MB send and 36 MB receive sizes, the per-client ceilings, and the 33 percent encoding note
• Attachment size exceeds the allowable limit error in Outlook — Microsoft's documentation of the 20 MB client cap for Internet accounts, the 10 MB Exchange default, and the MaximumAttachmentSize registry value
• Sending limits in Outlook.com — Microsoft's consumer guidance, currently stating 25 MB
• RFC 3463, Enhanced Mail System Status Codes — the definitions of the 5.2.3 and 5.3.4 codes in bounce messages

Client and tenant limits change with updates. If a delivery matters, test against your own mailbox rather than trusting a table — including this one.

Before you send

• ☐You know which of the limits above actually applies to your account type
• ☐The file is under 25 MB, not just under 35 MB, to survive encoding — or under 18 MB on a POP/IMAP or Outlook.com account
• ☐The recipient is not on a 10 MB corporate gateway
• ☐The PDF is not digitally signed, or it was compressed before signing
• ☐You kept the original file